1. Field of the Invention
The present invention relates to a method of forming a semiconductor thin film using high frequency and a source gas for formation of a semiconductor and, more particularly, to a method for forming an amorphous semiconductor thin film containing a microcrystalline layer, or a microcrystalline semiconductor thin film.
2. Related Background Art
In general, the plasma enhanced CVD process is widely used, for example, for fabrication of photovoltaic elements using amorphous silicon films or the like and is industrialized. In order for photovoltaic elements to meet the demand for electric power, however, the following fundamental requirements must be met by the photovoltaic elements used: photoelectric conversion efficiency thereof must be sufficiently high; they must have excellent stability; and they must be mass-producible.
To meet these requirements, fabrication of photovoltaic elements using amorphous silicon films or the like is intended to achieve improvements in electrical, optical, photoconductive, or mechanical characteristics and in fatigue characteristics or operating environment characteristics in repetitive use, and to allow mass production with repeatability by high-speed film formation, while also achieving increased film area and uniformity of the thickness and the quality of the film. These are problems to be solved in the future.
Many reports have been presented heretofore as to deposited film forming methods by the microwave plasma enhanced CVD process.
For example, the microwave-excited plasma CVD process using ECR is described in the following references:
"Chemical Vapor deposition of a-SiGe:H films utilizing a microwave-excited plasma," T. Watanabe, M. Tanaka, K. Azuma, M. Nakatani, T. Sonobe, T. Simada, Japanese Journal of Applied Physics, Vol. 26, No. 4, April, 1987, pp. L288-L290;
"Microwave-excited plasma CVD of a-Si:H films utilizing a hydrogen plasma stream or by direct excitation of silane," T. Watanabe, M. Tanaka, K. Azuma, M. Nakatani, T. Sonobe, T. Simada, Japanese Journal of Applied Physics, Vol. 26, No. 8, August, 1987, pp. 1215-1218, and so on.
Further, "Plasma vapor phase reaction apparatus" in Japanese Patent Application Laid-Open No. 59-16328 describes a method for depositing a semiconductor film by the microwave plasma CVD process. Furthermore, "Thin film forming method by microwave plasma" in Japanese Patent Application Laid-Open No. 59-56724 also describes a method for depositing a semiconductor film by the microwave plasma CVD process.
Another forming method of a deposited film with provision of a third electrode of a mesh shape between the anode and the cathode in the RF plasma enhanced CVD process is described in "Preparation of highly photosensitive hydrogenated amorphous Si--Ge alloys using a triode plasma reactor," A. Matsuda et al., Applied Physics Letters, Vol. 47 No. 10, 15, November, 1985 pp. 1061-1063.
Further, many electric power generation methods using the photovoltaic elements employ a method for connecting unit modules in series or in parallel to form a unit, thereby obtaining desired electric current and voltage. In that case each module is required to be free from wire breakage and short circuits. In addition, an important requirement is that there are no variations in output voltage and output current among the modules.
These require securing uniformity of the semiconductor layers themselves, since they are the most characteristic-decisive elements, at least in the stage of forming each unit module.
From the viewpoints of facilitating module design and simplifying module assembling steps, providing a semiconductor deposited film with excellent uniformity of features over a large surface area is required in order to enhance the mass producibility of photovoltaic elements and to achieve great reduction of production cost.
In the photovoltaic element, the semiconductor layers, which are important constituents thereof, compose a semiconductor junction such as the so-called pn junction or pin junction.
In the case of thin film semiconductors of amorphous silicon or the like being used, it is known that when a source gas containing an element being a dopant, such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6), is mixed with a main source gas such as silane, they undergo glow discharge decomposition to obtain a semiconductor film having a desired conductivity type, and such semiconductor films are stacked in order on a desired substrate, thereby readily attaining the aforementioned semiconductor junctions.
Proposed from this point as to formation of amorphous-silicon-based photovoltaic elements are methods for providing independent film forming chambers for formation of the respective semiconductor layers and for carrying out formation of the respective semiconductor layers in the associated film forming chambers.
Incidentally, U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD apparatus employing the roll-to-roll system. It is described therein that this apparatus can continuously fabricate the element having the semiconductor junctions by providing a plurality of glow discharge regions, setting a sufficiently long, flexible substrate of a desired width along a path in which the substrate successively passes through the glow discharge regions, and continuously conveying the substrate in the longitudinal direction thereof while depositing the semiconductor layers of the desired conductivity types in the respective glow discharge regions.
In the U.S. patent, gas gates are used in order to prevent the dopant gas used upon formation of each semiconductor layer from diffusing or mixing into other glow discharge regions. Specifically, the system employs means for separating the glow discharge regions from each other by slit-shaped separation passages and for forming a flow of scavenging gas, for example Ar, H.sub.2, or the like, in the separation passages.
Therefore, this roll-to-roll system is a system suitable for mass production of semiconductor elements, but, as described previously, further improvements in the photoelectric conversion efficiency, characteristic stability, and characteristic uniformity, and the reduction of production cost are necessary for widespread and volume use of photovoltaic elements.
Particularly, for the improvements in the photoelectric conversion efficiency and characteristic stability, it is a matter of course that the photoelectric conversion efficiency and characteristic deterioration rate of each unit module should be improved in the 0.1% order (corresponding to approximately 1.01 times by ratio). Further, when the unit modules are connected in series or in parallel to form a unit, a unit module of the minimum current or voltage characteristic out of the unit modules constituting the unit becomes a rate-limiting module so as to determine the characteristics of the unit. Therefore, it is very important to decrease characteristic variations, as well as to improve the average characteristics of each unit module.
It is thus demanded that the characteristic uniformity of the semiconductor layers themselves, being the most characteristic-decisive elements, be secured at the stage of forming the unit modules.
It is also strongly demanded that the yield be increased by decreasing defects of the semiconductor layers so as to avoid the breaking of wire and short circuits in each module, to decrease the production cost.
Attempts have been made in recent years to use microcrystalline silicon for the constituent layers of an amorphous silicon solar cell. For example, a pin element comprised of microcrystalline silicon produced by use of VHF of frequency 70 MHz and being free from optically induced deterioration is reported in "Intrinsic Microcrystalline Silicon (.mu.c-Si:H)--A Promising New Thin Film Solar Cell Material," J. Meier, S. Dubail, R. Fluckiger, D. Fisher, H. Keppner, A. Shah, Proceedings of First WCPEC; Dec. 5-9, 1994; Hawaii, pp. L409-L412.
In the case of the pin element free from optically induced deterioration made of microcrystalline silicon, however, the photoelectric conversion efficiency thereof is not more than 5% in the form of a cell of the single structure. When compared with the existing amorphous silicon solar cells, it cannot be regarded as good efficiency, even taking it into consideration that the amorphous silicon solar cells are subject to optically induced deterioration.
Further, it is generally known hitherto that the deposition rate of microcrystalline silicon film is as slow as not more than 1 .ANG./s and that the thickness of microcrystalline silicon layer necessary for exhibiting the sufficient characteristics as a solar cell is not less than 1 .mu.m. Therefore, the problem was that the throughput in mass production was not easy to increase.
As described above, microcrystalline silicon has the excellent feature of not suffering the optically induced deterioration on one hand, but it also had disadvantages of low conversion efficiency and inferior mass producibility when applied to solar cells.